Infrared from visible LEDs?

[DigiTan]

I noticed a strange phenomenia last summer while I was working on an infrared encoding/decoding project.

I had just programmed the both devices and powered up their breadboards while I searched for a clear-lens infrared LED. Since I also had visible clear-lens LEDs at the time, so I wound up inserting many of those first as I tried to ferret out the infrareds.

During the process I noticed the decoder was receiving RC5 codes from all of the visible LEDs, despite having used an IR detection module. While this only worked for much shorter ranges (around 6 feet versus the 30ft. I normally get indoors), the RC5 bit checking confirmed the data each time. The LEDs were red, white, and blue with the data modulating a 38kHz carrier.

How was the module able to recieve these signals? Is it possible the module's optical filter is wide enough to receive red and blue light? Or were these IR wavelengths coming from the LEDs? Also, I've noticed visible LEDs often appear white when viewed by my CMOS camara--which is an indicator of IR activity. Is it practical to use this IR for short range signaling?

 

[justDIY]

your ir receiver module is just a photodiode and some amplifier / filter parts... the photodiode is tuned to respond to a specific IR freq, but the nature of the junction makes it respond to any light of a higher wavelength as well.

i don't know about the "optical filter" ... most likely it's not an wavelength filter but a polarizing filter, to help keep out off-axis light

 

[hyedenny]

During the process I noticed the decoder was receiving RC5 codes from all of the visible LEDs........ Is it practical to use this IR for short range signaling?

It sounds like you already answered your own question!
LEDs emit light in a fairly broad spectrum, so its very possible, for example, that a red LED (635nm) emits a certain amount of 780nm IR light. Usually, the datasheet for an LED will have a graph of its intensity vs wavelength. Some "IR" detectors will also detect visible light unless you specifically order one that doesnt.

 

[Oznog]

LEDs don't emit on a broad spectrum at all. They're pretty specific!

However, as mentioned the IR receiver photodiode is usually is sensitive to visible light too. IR is made of lower energy photons than visible. Visible photons with more energy can still trigger it.

The filter is not a polarizer, it's a wavelength filter. Polarizing would suck because then your remote wouldn't work if you turned it 90 deg! But these filters don't have a really strong cutoff. I mean if you shine a flashlight at it the right away you can see through it, so it's not wiping out all non-IR light, just reducing it like 10x or more. If your visible LED is 3x closer (light intensity decrease with the square of distance) than the IR one, and you knew they both emitted the same amount of photons, both would have an equal signal out of the photodiode.

 

[hyedenny]

The specific wavelength of an LED is that which is measured at PEAK intensity of thel light emitted. That doesnt mean that other wavelengths are not emitted. A visible red LED can easily emit some IR light. Not at the same luminous intensity, but its still there. I guess my term "broad" is relative. A look at a datasheet will show that for a given LED, its spectrum is mostly in the +/-50nm range. Some are +/- 100nm. Even A UV LED will emit light all the way into the IR end, albeit at a very low intensity.... He DID say that it didnt work past 6 feet.

 

[Nigel Goodwin]

As already suggested, the receiver ISN'T IR specific, it also detects visible light - it has a crude IR pass filter in front of it, which reduces visible light, but doesn't block it.

That's why you use 38KHz modulation on the IR, to prevent visible light causing interference - although strong visible light can still swamp the receiver.

 

[DigiTan]

That sounds about right. After reading up on these IR pass materials, it looks like they also have a fairly low selectivity, and almost all of the ones I looked at could pass some visible light transmittance--even the high-quality filters they use for infrared photography. There were a few brands that did have a steep cutoffs though.

 

[Nigel Goodwin]

That sounds about right. After reading up on these IR pass materials, it looks like they also have a fairly low selectivity, and almost all of the ones I looked at could pass some visible light transmittance--even the high-quality filters they use for infrared photography. There were a few brands that did have a steep cutoffs though.

Most of them give a bit of a clue?, because you can see through them 8)